The invention relates to a form for a printing element which is one of a plurality of printing elements, separated from one another all around by lands, on a printing unit in an offset printing process such as, in particular, rotary offset printing, the printing element being in each case of a geometric, polygonal shape.
Since printing surface production began there has been a desire in the case of the offset printing process not to use photomechanical screens and instead to imitate the old, unscreened lithography or at least to achieve the closest possible approach to it. With the development of photolithography it became increasingly clear that though printing of relatively high quality could be obtained with screens of conventional design, it was not possible to imitate the old lithographic prints. Photomechanical screens, such as the gradar screen or magenta screen for example, were very good for the recently developed photolithography and also for printing surface production but screens of this type do suffer from certain technical shortcomings such as line breakdowns, moire patterns, secondary moire patterns and resetting. There is still a desire for lithography of the original kind, that is to say printing from stone, without a screen and with the only grain being that of the surface of the stone, to be made possible as a printing element for offset printing.
Consequently, the first very general aim exists of producing unscreened lithographs. The first idea was to use the colour grain of a transparency as a basis and to separate out this granulation by means of colour separation and use it as a printing element. This attempt failed due to the inadequacies of the photographic materials used. Some success was achieved with computer programs and PostScript page-description programs. However since these screening programs are subject to a controlled randomness generator, there is the problem that, the finer the resolution of the units selected, the more difficult is it to calculate the particular gradation curve. Because of this problem screens of this typexe2x80x94such as the Cristall, Diamond or Harlequin HD screensxe2x80x94are not suitable for rotary newspaper printing presses.
A return had to be made to conventional screening for offset printing. In rotary offset printing, a printing plate is mounted on a cylinder under tension. The cylinder co-operates with a further cylinder covered with a rubber blanket and the latter co-operates in turn with an impression cylinder over which the paper to be printed on is fed. The printing points on the printing plate are prepared in such a way that they repel water and accept the greasy printing ink. The non-printing points are prepared to have an affinity for water and they repel the greasy printing ink. In printing, the whole of the printing plate is first dampened, when only the non-printing points that have an affinity for water accept the water. The printing plate, which is thus damp in parts, then travels past inking rollers which transfer the grease-containing ink to the points on the printing plate that are not damp. The image to be printed is then transferred to the blanket cylinder and from there to the paper to be printed on.
The points in a printed image which are to be inked are split up into printing elements which cannot be seen by the naked eye. Each printing element is separated from the adjoining ones by lands. The lands are not inked. The greater the total area of the lands is in a dot in the image, the lighter the dot appears to be. The proportion that the inked area represents of the total area of a dot in the image defines a grey value, the half-tone value of the dot, and is normally given as a percentage.
By means of a screen having a plurality of printing elements distributed over it, it is possible, by varying the size of the printing elements, to simulate different half-tone values. It is known for the printing elements to take the form of squares, semicircular areas, or elliptical or other geometrical areas. Given the fineness that is possible in technical terms, the sharpness of the outlines in images has not been entirely satisfactory with the known forms of printing element. This is due to the fact that even when the printing elements are of only medium size, i.e. when the relevant region of the image is of medium half-tone value, there is some join-up between the dots, thus making the sharpness of outline and the graduations of the shadows in the printed image unsatisfactory. In the form of printing element that needs to be found, the dot join-up needs to be positioned to the xe2x80x9crearxe2x80x9d, in the direction where the shadow lies, as far as possible, and it needs to be possible for the new form of printing element to be controlled perfectly via gradation curves.
One solution to this problem is known from the form of printing element detailed in EP A 0 825 490. The imaginary screen cells are arranged in a chessboard pattern and in each of them is arranged a printing element, so that, if a tonal value, whatever it may be, remains constant across the screen cells, the shortest distance to the adjoining printing element is of at least approximately the same size for all points on the boundary lines of a printing element. In this case the printing element is to have a substantially rhomboid area, the boundary lines of which extend in curves in such a way that they form two diagonally opposed acute angles and two diagonally opposed rounded or obtuse angles. A printing element in the form of a flag is thus formed.
The advantage that this form of printing element and its arrangement in the screen has is that, purely in theory, join-up between the dots does not take place until a half-tone value of 100%. Although this form of printing element was developed specifically for four-colour printing, Moirxc3xa9 patterning continues to be a disadvantage in coloured illustrations. This phenomenon can only be avoided if, for another colour such as cyan, magenta or yellow, the printing elements are rotated about their centre axis from their normal arrangement for black, which is a very laborious operation in the regulating software.
The object on which the invention is based is to develop a new form of printing element with which even sharper reproduction and even finer, softer graduations in the shadows are possible, but in particular with which Moirxc3xa9 patterns do not occur in coloured prints, and of course while using only one and the same form for a printing element.
Starting from the experience gained with the printing element detailed in EP A 0 825 490, what is proposed as a way of achieving the stated object is that the given printing element be bounded in each case by S-shaped lines alone and the lines include an angle xe2x89xa690xc2x0 at all the corners of the printing element. It is particularly useful in this case if the printing element is bounded not only by three but rather by four lines, because what is then obtained is a sort of propeller figuration having gentle, swept S-shaped lines. A clearly definable geometrical form of printing element has thus been found.
The special advantage that this figuration for a printing element has is that if the propeller is mirrored in a direction transverse to an axis running through the centre and the points of reversal and between the opposing valley and hump lines, a printing element of the same size and form is obtained. If the mirrored printing element is then coloured in a different colour, preferably in magenta as a combination colour in the case of black and in yellow as a combination colour in the case of cyan, no Moirxc3xa9 patterns whatever occur, which is a considerable advantage for four-colour printing. Hence the rotation of the printing element which is needed with the flag printing element to avoid Moirxc3xa9 patterns is unnecessary when using this propeller printing element and in this case it is only the mirrored propeller that must be used for the other colour, something which can be set without any problems with the existing software. This printing element can be used for frequencies in the range from 152 to 304 lpi.
The printing element can of course be used for four-colour printing even without the mirroring. To avoid Moirxc3xa9 patterns in this case the printing element has to be rotated about its central axis as known previously.
So that the printed image can have different graduations of lightness, i.e. a varying tonal value, the printing elements need to be separated from the adjoining one by an area which is not inked. In the layout envisaged in the present application, these (white) areas are always in the form of lines of constant width, in such a way that the printing elements which are arranged next to one another in the screenxe2x80x94without being arranged in a chessboard patternxe2x80x94are so associated with one another that, at any tonal value, and even when the tonal value varies, the distances between the two adjoining sides and the next printing element are constant along the length of the side. This arrangement is a prerequisite for dot join-up between the printing elements which in each case adjoin one another not to occur until a half-tone value of approximately 100%. The result of this arrangement is soft differences in lightness and outlines of optimum sharpness in images produced with the present printing element.
Hence the present printing element can be used with advantage especially for coloured reproductions in newspaper, web-fed or sheet-fed offset printing
The form of the printing element according to the invention is shown by way of example in the drawings. In the drawings: